1. Field of Invention
The invention relates to a semiconductor ceramic device using a ceramic element which has a negative temperature coefficient of resistance.
2. Description of the Related Art
In a switching power source, for example, an overcurrent flows at the moment a switch is turned on. As a device for absorbing such an initial inrush current, a so-called NTC thermistor device is used. An NTC thermistor device has a high resistance at room temperature, and is characterized in that the resistance decreases as the temperature rises. This high resistance can suppress the level of an initial inrush current, and, when the temperature of the device is then raised by heat generated by the device itself, the resistance decreases so that the power consumption is reduced in a steady state. Conventionally, a spinel oxide is used as a ceramic element of such an NTC thermistor.
When such an NTC thermistor device is used to prevent an inrush current from flowing, the NTC thermistor device must have a low resistance in an elevated temperature state which is caused by the heat generated by the device itself. However, a conventional NTC device using a spinel oxide generally has a tendency that the B-value is small as the specific resistance is made low. Consequently, such a conventional NTC device has a problem in that the resistance cannot be decreased in an elevated temperature state to a sufficiently low level, thereby disabling the power consumption in a steady state to be reduced.
In Japanese Patent Publication (Kokoku) No. SHO 48-6352, etc., ceramics having a composition in which 20 mol% of Li.sub.2 O.sub.3 is added to BaTiO.sub.3 is proposed as an NTC thermistor device having a large B-value. However, this NTC thermistor device has a high specific resistance of 10.sup.5 .OMEGA..multidot.cm or higher at 140.degree. C., and hence there arises a problem in that the power consumption in a steady state is increased.
In contrast, a device using VO.sub.2 ceramics has resistance-sudden change characteristics in which the specific resistance is suddenly changed from 10 .OMEGA..multidot.cm to 0.01 .OMEGA..multidot.cm at 80.degree. C. Therefore, the device is excellent for use of preventing an inrush current from flowing. However the VO.sub.2 ceramic device has problems in that it is unstable, and that it must be rapidly cooled after a reducing firing process resulting in that its shape is restricted to a bead-like one. Since the allowable current of the device is as low as several tens of milliamperes, there arises a problem in that the device cannot be used in an apparatus such as a switching power source where a large current flows.